Tldr: uniforms are an anachronism, used as means of constraining and controlling kids, and reminding them who is in charge.

Very long; reading if you like:

Using uniforms to disguise the differences between rich and poor was a post War idea that made a lot of sense in the UK. The introduction of public schools at the time brought kids together from a wide range of backgrounds just when those differences were very obvious. It all made a lot of sense. Also, English private schools had famously used uniforms as it created a certain image that they wanted. To a degree this is now true of public schools, and the "tradition" of wearing uniforms is often given as a reason to keep them.

In the UK, in 2024, there are always kids wearing uniforms that are unwashed, need repairing, or just the wrong size. But even without those signs the kids all know who is rich and who is not.

If you take kids out of their uniforms and let them wear their preferred clothes, the differences between become very hard to spot. Back in 1942 this was not true, but today you'd be hard pushed to tell who is rich and who is not making the original purpose irrelevant.

To setup a study of the impact of uniforms is logistically impossible, however all studies that look at educational outcomes found no correlation between uniform and exam results. This seems to disprove the claim that uniforms allow kids to do better in class.

Today, schools like to have uniforms for a few reasons. One, they claim it is part of the school identity, which is true to a degree. But it came about due to national hardship, and is a reminder of times we no longer need. Two, school rules on uniforms have always existed but have become more strict in recent years (speaking as a UK parent of 3 kids aged 12, 16, & 25). In the school my kids attend(ed), they will send letters home and punish kids for uniform related infractions the moment they happen. However, they don't do the same for education related issues. It took 2 years for them to report the issues my daughter was having, and they seemed more concerned with her earnings than her school work when we spoke with them.

Dropping uniforms would reduce the load on teachers who would not need to constantly monitor and punish kids for wearing the wrong shoes, or coats, or bags, or having earrings even. There would still be some rules but they would allow far more latitude for self expression and freedom that kids in 2024 require. And this is an important point. For good or bad, modern children are exposed to information, news, behaviours, and attitudes from around the world, from a very early age. They are also going through the universal issues of adolescence. We need to accept they need freedom to express themselves, not find ways to constrain them.

On a side note, it is my own view that some schools are scared of giving kids freedom to choose, and the uniform rules are intended as a way to make them "conform" to a conservative standard. Uniform rules are effectively a stick they can smack kids with. Take it away and you take their power.